Last April, about a month after the Fukushima nuclear accident, concentrations of cesium-137 in the ocean near the plant peaked at 50 million times above normal levels, according to a study by the Woods Hole Oceanographic Society.

Last May, Woods Hole senior scientist Ken Buesseler told this page of his planned expedition to study the effects of radiation on the ocean.

“The release of radioactivity from Fukushima–both as atmospheric fallout and direct discharges to the ocean–represents the largest accidental release of radiation to the ocean in history,” according to a National Science Foundation synopsis of the study.

“Concentrations of cesium-137, a radioactive isotope with a 30-year half-life, at the plants’ discharge points to the ocean peaked at more than 50 million times normal/previous levels. Concentrations 18 miles offshore were higher than those measured in the ocean after the Chernobyl accident 25 years ago.”

Despite the alarming concentrations, the radiation rapidly diluted in the currents northwest of the plant, the scientists contend, and they should pose little or no threat to human and marine life. However, little is known about the effect of radiation on the lowest levels of the coastal ecosystem.

“We don’t know how this might affect benthic [bottom dwelling and subsurface] marine life, and with a half-life of 30 years, any cesium-137 accumulating in sediments or groundwater could be a concern for decades to come,” Buesseler said.

The radionuclides peaked in April when Toyko Electric Power was using water to cool exposed reactor cores and spent fuel and discharging the contaminated water into the ocean. The levels dropped dramatically in May when crews began capturing the contaminated water for treatment.

But even in July, radiation levels were still 10,000 times higher than levels measured in 2010 off the coast of Japan, the study found, and the plants “remain a significant source of contamination to the coastal waters off Japan.”

“There is currently no data that allow us to distinguish between several possible sources of continued releases,” says Buesseler.

“These most likely include some combination of direct releases from the reactors, or storage tanks or indirect releases from groundwater beneath the reactors or coastal sediments, both of which are likely contaminated from the period of maximum releases.”